EXTINCTIONS × CLIMATE

The First Mass Extinction

~445 million years ago

Phanerozoic · Paleozoic · Ordovician

The first of the five great crises, and the only one whose immediate cause was cold. Tropical seas that had held at 32 to 37 degrees cooled by about five, ice sheets grew to match the peak of the last Ice Age and probably to exceed it, sea level fell by about a hundred metres, and the shallow shelf seas where almost everything lived drained away. Toxic water from the depths rose onto what was left of the shelves. It killed in two pulses.

Why it matters

Species died in enormous numbers, but few whole branches of the tree of life were lost and the structure of the ecosystem came through: a massive taxonomic extinction, a comparatively mild ecological one. What it cost was time. Diversity did not return to its earlier level for at least thirty-five million years.

Dating & uncertainty

The two pulses fall inside the Hirnantian itself, one near its beginning and one in its middle. Their relative severity is disputed, as is the trigger: cooling, ocean chemistry and volcanic mercury spikes each have their advocates.

Sources

1. Finnegan et al. (2011), Science · The magnitude and duration of Late Ordovician-Early Silurian glaciation
2. Rasmussen et al. (2019), PNAS · Cascading trend of Early Paleozoic marine radiations paused by Late Ordovician extinctions
3. Harper et al. (2014), Gondwana Research · End Ordovician extinctions: a coincidence of causes
4. Harper (2024), National Science Review · Late Ordovician Mass Extinction: Earth, fire and ice

Earth history as one day

21:38:512 h 21 min before midnight

if Earth’s 4.54 billion years were compressed into a single day